Not often I try to study the feel of my own mouth when I speak, however…

“Th” like “the” is more the tip of the tongue to the back of the upper front teeth. Feels like to the bottom edge of the front teeth to be precise. (Which is probably why some people end up with a speech problem by sticking their tongue too far past their teeth.)

The “d” like “dog is more like the tongue to the roof of the mouth just behind the teeth. Does not feel like tip of tongue, but just a little further back.

Edit: This page has related diagrams and info at the bottom of the page. http://efl.htmlplanet.com/phono.htm

I don’t stick my tongue out. I touch the tip of my tongue to the back of my teeth. The very, very tip sort of sticks out past the teeth.

I also asked two other adults in the room to try it. They both do the same thing. One says “th” with the tip of her tongue basically even with the front of her teeth. The other says it with her tongue very, very slightly sticking past the front of her teeth. I’m between the two.

And by “behind”, I mean that the tip is between the top and bottom sets of teeth.